1. Field of the Invention
This invention relates to radio receivers and, more particularly, to noise estimation within frequency modulation (FM) receivers.
2. Description of the Related Art
Frequency modulation (FM) radio is widely used for radio programming. One reason FM became more popular for music broadcasts than, for example, amplitude modulation (AM), is in many cases FM reception is less affected by noise than AM. In an FM signal, the transmitted carrier signal frequency is varied by the frequency of the information (e.g., at an audio rate) within a given bandwidth. In an AM signal, the transmitted carrier signal amplitude is varied by the frequency of the information (e.g., at an audio rate), thereby creating an envelope that is defined by the information frequency. As such, a significant drawback to AM signals is the amount of noise that may be present in the received signal. In many FM receivers, the effects of received noise may be reduced using various well-known methods such as limiting or over-driving. In these methods, the received signal may be over-driven such that the amplifiers begin to clip. Since noise in an FM signal may be “riding” on the peaks of the received signal, much of the noise may be removed, while the information carried by the signal remains.
Modern FM receivers include many features such as soft mute, seek, and blend, for example, which may enhance a users listening experience. To effectively implement these types of features, an estimate of the signal-to-noise ratio (SNR) may be desired. However, depending on such parameters as the environment and the frequency, the signal strength and noise floor may vary, thus making accurate SNR estimations difficult.